Urban Meteorology and Air Quality

Cities change the air above them: they heat it and drive breeze circulations, intensify heat waves, and trap pollutants between buildings. My work in urban meteorology ranges from analytic theory of urban breeze circulations and their interaction with mountain slope winds, to the urban heat island and its interaction with heat waves, and observations of turbulence and air pollutants in urban street canyons.

Figure. Schematic of the urban atmosphere: the urban heat island drives an urban breeze circulation that interacts with mountain and park breezes, while traffic emissions disperse in street canyons and form secondary ozone downwind.

Urban breeze circulations

Linear theory with diurnally varying heating shows how basic-state wind, the Coriolis force and friction shape the urban breeze and its interaction with mountain slope winds. At higher latitude, the shorter inertial period and rotation modify when and how strongly the reversed urban breeze circulation appears.

Figure. Vertical velocity (black lines), velocity vectors, buoyancy (shading) and kinematic pressure (gray lines) at 0200 LT (left) and 1700 LT (right) for (a, b) urban thermal forcing only, (c, d) mountain thermal forcing only and (e, f) mountain mechanical forcing only (Seo et al. 2017, Theor. Appl. Climatol.).

Urban heat island and heat waves

With colleagues, I analyzed the urban heat island (UHI) of Dhaka, Bangladesh, over 1995–2019 and its interaction with heat waves. The UHI intensity has increased by about 0.21 °C per decade. Under heat waves the UHI is synergistically intensified, in both daytime and nighttime in the pre-monsoon season and at night in the monsoon season, with decreases in relative humidity and cloud fraction favoring the interaction (Tabassum et al. 2024, APJAS).

Figure. Box plots of daytime (a, d) and nighttime (b, e) UHI intensity and diurnal variations (c, f) under heat waves (red) and non-heat waves (blue) in the pre-monsoon (top) and monsoon (bottom) seasons in Dhaka (Tabassum et al. 2024).

Air quality in urban street canyons

With colleagues, I measured turbulence and ozone at rooftop and sidewalk sites in a high-rise building area of Seoul, and related rooftop and on-road concentrations of traffic-related pollutants in a busy street canyon to the ambient wind. How pollutants disperse inside the canyon, and how strongly rooftop and road levels are linked, depends on whether the wind blows across or along the canyon (Kwak et al. 2016, Environ. Pollut.; Park et al. 2015, SOLA).

Figure. Schematics of in-canyon pollutant dispersion processes for (a) cross-canyon and (b) along-canyon wind directions; arrow width indicates the relative importance of each process (Kwak et al. 2016).